1. Field of the Invention
The present invention relates to a composition and method for diffusion coating ferrous metals, and more particularly to a method for diffusion coating ferrous metals using a pulverous coating composition including chromium and ferrochromium.
2. Description of Related Art
Carbon steels are frequently used in various industries due to their high plasticity, that is, their ability to deform inelastically without rupture at high stresses. This high plasticity, in turn, makes carbon steels relatively easy to machine, process and treat.
One drawback to the use of carbon steels and cast iron is their low wear resistance and low corrosion resistance. For this reason, these materials are often subjected to surface treatments to increase their resistance to wear and corrosion. Prior investigations have shown that the diffusion coating of parts with alloys of the transition metals, especially alloys of chromium, produces dense protective layers which are connected reliably to the base metals of the parts.
Various compositions have been proposed for the diffusion coating of ferrous base metals. One such composition has the following components:
Chromium 50 wt % Aluminum Oxide 43-45 wt % Ammonium Chloride 5-7 wt %.
The diffusion coating of carbon-containing ferrous base metals with this composition produces a surface layer comprising a solid solution of chromium in iron with a discontinuous chromium carbide phase.
Such surface layers have been found to be susceptible to fracture, leading to a relatively short coating life. This is believed to be due to the failure of the coating. Such surface layers also have been found to be insufficiently resistant to corrosion, especially in weak acidic or chloridic solutions at high temperatures.
Another proposed diffusion coating composition uses ferrochromium:
 Ferrochromium 70 wt % Aluminum Oxide 29 wt % Chromium Ammonia 1 wt %
The wear resistance of ferrous base metals diffusion-coated with this composition is very low.
Various microadditions, such as boron and molybdenum, have been proposed for increasing the wear resistance of chromium-based diffusion coatings. For example, German Patent No. 36 04 309 proposed the following composition for use in diffusion coating metal:
 Chromium 67 wt % Molybdenum Boride 3 wt % Aluminum Oxide 29 wt % Ammonium Chloride 1 wt %.
Such microadditives tend to enhance the formation of a continuous upper surface carbide layer and thereby meaningfully increase the corrosion resistance and wear resistance of the finished part.
Nonetheless, coatings formed on carbon-containing ferrous base metals from chromium-based compositions including boron or molybdenum microadditives lack sufficient resistance to aggressively corrosive solutions such as calcium chloride, sulfur-containing petroleum and mineral oil. Furthermore, prior art coating compositions have required relatively large amounts of these relatively expensive microadditives (from approximately 2-5 wt %) which significantly increased the cost of the coatings.
Pure tantalum is widely known to be inert with respect to many corrosive agents, including hydrochloric, nitric and acetic acids; lye; sea water; and chloridic solutions. Tantalum carbide, which is synthesized by the direct carbidization of tantalum powder and soot, or by the reaction of tantalum oxide with carbon at 1900.degree. C. in an inert gas atmosphere, is known to possess high hardness and high resistance to corrosion except at elevated temperatures.
Tantalum metal is widely used in sheet form in the manufacture of different kinds of apparatus including vessels, heaters, steam condensers and pipe heat exchangers. Tantalum is rarely used for coating other metals, however. Though tantalum coatings might be formed by explosion or by precipitation from the vapor phase, these processes are expensive and do not guarantee sufficiently continuous, unbroken coverage of the base metals.
Soviet Author's Certificate No. 10 66 537 proposed a coating composition including 4 wt % nickel, 4 wt % chromium and 17 wt % tantalum. Coatings formed from this composition showed improved wear resistance but were relatively brittle. Furthermore, such coatings did not provide sufficient resistance to corrosion.
German Patent No. 42 38 220 proposed a composition for the diffusion coating of ferrous metals such as cast iron:
 Chromium 50-60 wt % Tantalum Carbide 0.75-2.5 wt % Ammonium Chloride 1-3 wt % Aluminum Oxide Remainder
This composition produced coatings with high wear and corrosion resistance. Nonetheless, the relatively large percentages of pure chromium and tantalum carbide required by the formulation raised the costs of the coated parts.
There remains a need in the art for an economical diffusion coating method and composition for forming effective wear and corrosion resistant surface layers over ferrous base metals.